1. Field or the Invention
The present invention relates to an antenna unit which receives radio waves transmitted from a satellite, and particularly to an antenna unit suitable for a car antenna attached onto an automotive roof.
2. Description of the Related Art
In car navigation systems, the present position is specified by speed and running distance of the car, while, in order to improve accuracy of position, radio waves transmitted from a GPS (global positioning system) satellite are received and the present position is specified also by the position data obtained from the received radio waves.
Recently, in the USA, it has been promoted to provide digital radio broadcasting by radio waves transmitted from an earth satellite. Also, in a digital radio receiving system which receives such the digital radio broadcasting, an antenna for receiving radio waves transmitted from the satellite is still required, and a so-called satellite radio broadcasting reception antenna is used.
The radio wave from the satellite, since a high-frequency band is used, has high directivity. Accordingly, in the car navigation system and the digital radio receiving system, in order to receive the radio waves from the satellite in a good reception state, it is necessary to attach a reception antenna onto a top surface (i.e., roof) of the car. Therefore, for the antenna unit for receiving the radio waves from the satellite, high atmospheric corrosion resistance and waterproof property are necessary.
As a conventional waterproof countermeasure in this kind of antenna unit, it is general to use a gasket (packing) composed of rubber, and attachment structure as shown in FIG. 6 is widely adopted. Namely, a bottom surface of a cover member 101 in which an antenna module for receiving radio waves transmitted from the satellite is housed is sealed by a bottom plate 103 to which a gasket 102 is attached, and the antenna unit is set onto a roof panel 104 of a car thereby to prevent entry of water. At this time, as shown in FIG. 7, a slit 102a is provided for the gasket 102 thereby to provide double structure, and an end 101a on the bottom side of the cover member 101 is inserted into this slit 102a thereby to secure the sealing state.
However, in case that the aforesaid waterproof structure is adopted, the surroundings near the bottom surface of the cover member 101 are covered with a cover part 102b of the gasket 102 like a skirt, and such mischief that this skirt-like portion is pulled by a hand has been frequently reported. In case that the cover part 102 of the gasket 102 is pulled, there is fear that a waterproof function is damaged, which is a large problem.
Further, the state in which the surroundings near the bottom surface of the cover member 101 are covered with the cover part 102b like the skirt mare the appearance greatly. Further, also from a viewpoint of the waterproof function, there is a problem that water easily enters the inside of the cover member 101 from a gap between the cover part 102b and the cover member 101, and improvement of this problem is desired.